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Timeline of a nuclear explosion in pictures
Pictures from several detonations as examples of an atomic explosions anatomy
Photographers and reporters gather near Frenchman Flat to observe the Priscilla nuclear test; June 24, 1957
Nuclear Chemistry Behind the Explosion
Atomic bombs are made up of a fissile element, such as uranium, that is enriched in the isotope that can sustain a fission nuclear chain reaction. When a free neutron hits the nucleus of a fissile atom like uranium-235 (235U), the uranium splits into two smaller atoms called fission fragments, plus more neutrons. Fission can be self-sustaining because it produces more neutrons with the speed required to cause new fissions. This creates the chain reaction.
The uranium-235 content of "weapons-grade" uranium is generally greater than 85 percent, though inefficient weapons, deemed "weapons-usable," can be made of 20 percent enriched uranium. The very first uranium bomb, Little Boy, dropped on Hiroshima in 1945, used 64 kilograms of 80 percent enriched uranium.
In fission weapons, a mass of fissile material, either enriched uranium or plutonium, is assembled into a supercritical mass—the amount of material needed to start an exponentially growing nuclear chain reaction. This is accomplished either by shooting one piece of sub-critical material into another, termed the "gun" method, or by compressing a sub-critical sphere of material using chemical explosives to many times its original density, called the "implosion" method.
The implosion method is considered more sophisticated than the gun method and only can be used if the fissile material is plutonium. The inherent radioactivity of uranium will then release a neutron, which will bombard another atom of 235U to produce the unstable uranium-236, which undergoes fission, releases further neutrons, and continues the process.The uranium atom can split any one of dozens of different ways, as long as the atomic weights add up to 236 (uranium plus the extra neutron). The following equation shows one possible split, namely into strontium-95 (95Sr), xenon-139 (139Xe), and two neutrons (n), plus energy:
The immediate energy release per atom is about 180 million electron volts (Me). Of the energy produced, 93 percent is the kinetic energy of the charged fission fragments flying away from each other, mutually repelled by the positive charge of their protons. This initial kinetic energy imparts an initial speed of about 12,000 kilometers per second.
However, the charged fragments' high electric charge causes many inelastic collisions with nearby nuclei, and thus these fragments remain trapped inside the bomb's uranium pit. Here, their motion is converted into X-ray heat, a process which takes about a millionth of a second. By this time, the material in the core and tamper of the bomb is several meters in diameter and has been converted to plasma at a temperature of tens of millions of degrees. This X-ray energy produces the blast and fire which are normally the purpose of a nuclear explosion.
Source: Boundless. “The Atomic Bomb.” Boundless Chemistry. Boundless, 08 Aug. 2016. Retrieved 10 Nov. 2016 from The Atomic Bomb - Boundless Open Textbook
Detonation: The Actual Time of the Reaction
Neutron travels at speeds of about 10 million meters per second, or about 3% the speed of light. The characteristic time for a generation is roughly the time required to cross the diameter of the sphere of fissionable material.
A critical mass of uranium is about the size of a softball (0.1 meters).
The time the neutron would take to cross the sphere is:
OR
The complete process of a bomb explosion is about 80 times this number, or slightly less than a microsecond or .0000008 seconds.
The 80 times represents how many fissions generations occurred in the Trinity test bomb but 99.99% of the energy is released in the last 10 generations, in total .00000008 seconds. This time was informally known as a 'shake' ("as fast as the shake of a lamb's tail") by the physicists at Los Alamos.
.00000008 seconds
Immediately after the explosion time, the temperature of the weapon material is several tens of million degrees and the pressures are estimated to be many million atmospheres. As a result of numerous inelastic collisions, part of the kinetic energy of the fission fragments is converted into internal and radiation energy. Some of the electrons are removed entirely from the atoms, thus causing ionization, others are raised to higher energy (or excited) states while still remaining attached to the nuclei. Within an extremely short time, perhaps a hundredth of a microsecond or so, the weapon residues consist essentially of completely and partially stripped (ionized) atoms, many of the latter being in excited states, together with the corresponding free electrons. The system then immediately emits electromagnetic (thermal) radiation, the nature of which is determined by the temperature. Since this is of the order of several times 107 degrees, most of the energy emitted within a microsecond or so is in the soft X-ray region
H (maximum for local fallout)
where H feet is the maximum value of the height of burst for which there will be appreciable local fallout. This expression is plotted above. For an explosion of 1,000 kilotons, i.e., 1 megaton yield, it can be found from the above graph that significant local fallout is probable for heights of burst less than about 2,900 feet. It should be emphasized that the heights of burst estimated in this manner are approximations only, with probable errors of +30 percent. Furthermore, it must not be assumed that if the burst height exceeds the value in the graph there will definitely be no local fallout. The amount, if any, maybe expected, however, to be small enough to be tolerable under emergency conditions.
H (maximum for local fallout)
where H feet is the maximum value of the height of burst for which there will be appreciable local fallout. This expression is plotted above. For an explosion of 1,000 kilotons, i.e., 1 megaton yield, it can be found from the above graph that significant local fallout is probable for heights of burst less than about 2,900 feet. It should be emphasized that the heights of burst estimated in this manner are approximations only, with probable errors of +30 percent. Furthermore, it must not be assumed that if the burst height exceeds the value in the graph there will definitely be no local fallout. The amount, if any, maybe expected, however, to be small enough to be tolerable under emergency conditions.
.001 seconds
It is apparent that the kinetic energy of the fission fragments, constituting some 85 percent of the total energy released, will distribute itself between thermal radiation, on the one hand, and shock and blast, on the other hand, in proportions determined largely by the nature of the ambient medium. The higher the density of the latter, the greater the extent of the coupling between it and the energy from the exploding nuclear weapon. Consequently, when a burst takes place in a medium of high density, e.g., water or earth, a larger percentage of the kinetic energy of the fission fragments is converted into shock and blast energy than is the case in a less dense medium, e.g., air. At very high altitudes, on the other hand, where the air pressure is extremely low, there is no true fireball and the kinetic energy of the fission fragments is dissipated over a very large volume. In any event, the form and amount in which the thermal radiation is received at a distance from the explosion will depend on the nature of the intervening medium.
At very early times, beginning in less than a microsecond, an "inner" shock wave forms driven by the expanding bomb debris. This shock expands outward within the isothermal sphere at a velocity exceeding the local acoustic velocity. The inner shock overtakes and merges with the outer shock at the fireball front shortly after hydrodynamic separation. The relative importance of the debris shock wave depends on the ratio of the yield to the mass of the exploding device and on the altitude of the explosion
.001 seconds
.001 seconds
Explosion at .001 seconds. The support tower in the image above provides a convenient size scale. Most of the above images capture the fireball when it is 100 feet in diameter.
At this stage of the detonation the surface of the fireball has a temperature of 20,000 degrees, three times hotter than the sun's surface. The spikes in the above images are a result from the guide wires supporting the tower on which the bomb was located absorbing enough heat to turn into light emitting plasma. Because thermal radiation travels faster than the fireball, the spikes extend out ahead of it.
a few milliseconds old you can see the shockwave hitting the desert floor in front of the fireball and bouncing back. Various trucks and tanks are scattered on the desert floor for the test
Just touching the ground
.010 seconds The uneven surface of these explosions has been attributed to variations in the bomb construction, thickness and materials as they are vaporized and turn into this expanding shell of plasma, fire and various gasses.
A primary form of energy from a nuclear explosion is thermal radiation. Initially, most of this energy goes into heating the bomb materials and the air in the vicinity of the blast. Temperatures of a nuclear explosion reach those in the interior of the sun, about 100,000,000° Celsius, and produce a brilliant fireball.
Two pulses of thermal radiation emerge from the fireball. The first pulse, which lasts about a tenth of a second, consists of radiation in the ultraviolet region. The second pulse which may last for several seconds, carries about 99 percent of the total thermal radiation energy. It is this radiation that is the main cause of skin burns and eye injuries suffered by exposed individuals and causes combustible materials to break into flames.
For some time the fireball continues to grow in size at a rate determined by the propagation of the shock front in the surrounding air. During this period the temperature of the shocked air decreases steadily so that it becomes less opaque. Eventually, it is transparent enough to permit the much hotter and still incandescent interior of the fireball, i.e., the isothermal sphere, to be seen through the faintly visible shock front. The onset of this condition at about 15 milliseconds (0.015 second) after the detonation of a 20-kiloton weapon, for example, is referred to as the "breakaway."
Following the breakaway, the visible fireball continues to increase in size at a slower rate than before, the maximum dimensions being attained after about a second.
.016 seconds
The Trinity explosion, 16 ms after detonation. The viewed hemisphere’s highest point in this image is about 200 metres (660 ft) high.
As the fireball cools, the transfer of energy by radiation and radiative growth become less rapid because of the decreasing mean free path of the photons. When the average temperature of the isothermal sphere has dropped to about 300,000°C, the expansion velocity will have decreased to a value comparable to the local acoustic (sound) velocity. At this point, a shock wave develops at the fireball front and the subsequent growth of the fireball is dominated by the shock and associated hydrodynamic expansion. The phenomenon of shock formation is sometimes called "hydrodynamic separation." For a 20-kiloton burst it occurs at about a tenth of a millisecond after the explosion when the fireball radius is roughly 40 feet
at .o25 seconds … having expanded to a 1000 ft in diameter the double shock wave is apparent on the expanding edges
Trinity test ground zero after blast.
The original test report from Los Alamos Trinity test

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Hydrogen bomb sequence coming.
Redwing H-Bomb detonation on a barge at Bikini atoll in Operation Redwing in 1956.
A Question of Scale
Castle Bravo Sequence
On March 1, 1954, it was the highest yield test in the United States’ highest-yield nuclear test series, exploding with a force of 15 million tons of TNT.

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Castle BRAVO, 3.5 seconds after detonation, photo taken from a distance of 75 nautical miles from ground zero, from an altitude of 12,500 feet.
Castle BRAVO, 62 seconds after detonation. “This image was take at a distance of 50 [nautical miles] north GZ from an altitude of 10,000 feet. The lines running upward to the left of the stem and below the fireball are smoke trails from small rockets. At this time the cloud stem was about 4 mi in diameter.”
Castle BRAVO, 16 minutes after detonation, seen from a distance of 50 nautical miles, at an altitude of 10,000 feet.
This is a work in progress … I will be adding to it
Trinity science details taken from Scientific Aspects of Nuclear Explosion Phenomena
For a current assessment of the threat of nuclear destruction go here Allen E Hall's answer to How destructive is the World's nuclear arsenal?
Allen E. Hall